mm: add shmem_zero_setup_desc()

Add the ability to set up a shared anonymous mapping based on a VMA
descriptor rather than a VMA.

This is a prerequisite for converting to the char mm driver to use the
mmap_prepare hook.

Link: https://lkml.kernel.org/r/d918151[e-mail]
Signed-off-by: Lorenzo Stoakes <[e-mail]>
Reviewed-by: Jason Gunthorpe <[e-mail]>
Cc: Alexander Gordeev <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Andreas Larsson <[e-mail]>
Cc: Andrey Konovalov <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Baolin Wang <[e-mail]>
Cc: Baoquan He <[e-mail]>
Cc: Chatre, Reinette <[e-mail]>
Cc: Christian Borntraeger <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: Dan Williams <[e-mail]>
Cc: Dave Jiang <[e-mail]>
Cc: Dave Martin <[e-mail]>
Cc: Dave Young <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: David S. Miller <[e-mail]>
Cc: Dmitriy Vyukov <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Guo Ren <[e-mail]>
Cc: Heiko Carstens <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: James Morse <[e-mail]>
Cc: Jan Kara <[e-mail]>
Cc: Jann Horn <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Kevin Tian <[e-mail]>
Cc: Konstantin Komarov <[e-mail]>
Cc: Liam Howlett <[e-mail]>
Cc: "Luck, Tony" <[e-mail]>
Cc: Matthew Wilcox (Oracle) <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Mike Rapoport <[e-mail]>
Cc: Muchun Song <[e-mail]>
Cc: Nicolas Pitre <[e-mail]>
Cc: Oscar Salvador <[e-mail]>
Cc: Pedro Falcato <[e-mail]>
Cc: Robin Murohy <[e-mail]>
Cc: Sumanth Korikkar <[e-mail]>
Cc: Suren Baghdasaryan <[e-mail]>
Cc: Sven Schnelle <[e-mail]>
Cc: Thomas Bogendoerfer <[e-mail]>
Cc: "Uladzislau Rezki (Sony)" <[e-mail]>
Cc: Vasily Gorbik <[e-mail]>
Cc: Vishal Verma <[e-mail]>
Cc: Vivek Goyal <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Will Deacon <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/shmem_fs.h

···
 					    unsigned long flags);
 extern struct file *shmem_file_setup_with_mnt(struct vfsmount *mnt,
 		const char *name, loff_t size, unsigned long flags);
-extern int shmem_zero_setup(struct vm_area_struct *);
+int shmem_zero_setup(struct vm_area_struct *vma);
+int shmem_zero_setup_desc(struct vm_area_desc *desc);
 extern unsigned long shmem_get_unmapped_area(struct file *, unsigned long addr,
 		unsigned long len, unsigned long pgoff, unsigned long flags);
 extern int shmem_lock(struct file *file, int lock, struct ucounts *ucounts);

mm/shmem.c

···
 }
 EXPORT_SYMBOL_GPL(shmem_file_setup_with_mnt);
 
-/**
- * shmem_zero_setup - setup a shared anonymous mapping
- * @vma: the vma to be mmapped is prepared by do_mmap
- */
-int shmem_zero_setup(struct vm_area_struct *vma)
+static struct file *__shmem_zero_setup(unsigned long start, unsigned long end, vm_flags_t vm_flags)
 {
-	struct file *file;
-	loff_t size = vma->vm_end - vma->vm_start;
+	loff_t size = end - start;
 
 	/*
 	 * Cloning a new file under mmap_lock leads to a lock ordering conflict
···
 	 * accessible to the user through its mapping, use S_PRIVATE flag to
 	 * bypass file security, in the same way as shmem_kernel_file_setup().
 	 */
-	file = shmem_kernel_file_setup("dev/zero", size, vma->vm_flags);
+	return shmem_kernel_file_setup("dev/zero", size, vm_flags);
+}
+
+/**
+ * shmem_zero_setup - setup a shared anonymous mapping
+ * @vma: the vma to be mmapped is prepared by do_mmap
+ * Returns: 0 on success, or error
+ */
+int shmem_zero_setup(struct vm_area_struct *vma)
+{
+	struct file *file = __shmem_zero_setup(vma->vm_start, vma->vm_end, vma->vm_flags);
+
 	if (IS_ERR(file))
 		return PTR_ERR(file);
 
···
 		fput(vma->vm_file);
 	vma->vm_file = file;
 	vma->vm_ops = &shmem_anon_vm_ops;
+
+	return 0;
+}
+
+/**
+ * shmem_zero_setup_desc - same as shmem_zero_setup, but determined by VMA
+ * descriptor for convenience.
+ * @desc: Describes VMA
+ * Returns: 0 on success, or error
+ */
+int shmem_zero_setup_desc(struct vm_area_desc *desc)
+{
+	struct file *file = __shmem_zero_setup(desc->start, desc->end, desc->vm_flags);
+
+	if (IS_ERR(file))
+		return PTR_ERR(file);
+
+	desc->vm_file = file;
+	desc->vm_ops = &shmem_anon_vm_ops;
 
 	return 0;
 }